The ERK mitogen activated protein kinases (MAPK) pathway has been implicated in a number of skeletal disorders. Our recent genetic experiments in mice have indicated that the ERK MAPK pathway plays critical roles in the regulation of chondrocyte differentiation and osteoblast differentiation. We hypothesize that 1) Krox20 expression is regulated by ERK1/ERK2 through its MAPK response element via DNA binding complex containing direct ERK1/ERK2 substrates, 2) ERK1 and ERK2 regulate differentiation of perichondrial/periosteal cells in a cell autonomous manner, 3) ERK1 and ERK2 in the perichondrium/periosteum and osteoblasts regulate chondrocyte phenotype in the adjacent epiphyseal cartilage. We will test these hypotheses by pursuing the following Specific Aims: Aim 1. Identify mechanisms whereby ERK1 and ERK2 regulate Krox20 expression in skeletal cells, Aim 2. Determine whether the effect of ERK1 and ERK2 inactivation on the perichondrium/periosteum is cell autonomous, Aim 3. Determine how loss of ERK1 and ERK2 in committed osteoblasts affects skeletal development. These experiments will provide novel insights into the roles of ERK1 and ERK2 in mesenchymal cell differentiation and skeletal development. PUBLIC HEALTH RELEVANCE: Human mutations in the molecules in the ERK MAPK pathway have been identified in a number of skeletal syndromes. This study identifies novel regulatory mechanisms of bone formation by ERK1 and ERK2. The identification of the regulatory mechanisms will provide much needed information for controlling bone formation in various skeletal disorders such as osteoporosis and genetic skeletal syndromes.